Intake manifold and port

jseng1 said:
Check the cross section at its small point. 28mm is too small and will choke down performance above 1/2 throttle with twin carbs. Go higher and wider where you can. When I ported my first head from 28+mm to 30+ I was amazed at the performance increase.
Click to expand...

This is talking BSA ports again but I can usually cut the 34mm port to 38-39mm wide at the guide. On the 38mm port as close as I could copy the XR750 port is around 44mm wide from memory but it goes through so JBweld between fins is easy but a plate stuck on from the pushrod tunnel side needs shaping and is difficult. JBweld over a hole into the pushrod tunnel ends up cracking on its own, a thin alloy plate makes it work though a smooth transition between alloy and JBweld in the port is challenging because the epoxy sands away quicker. Welding is pretty impossible even though there is heaps of room before interfering with the pushrods. Some BSA heads they made thicker when they ported the 68 Spitfire head which was machined different. Deeper bowl and wider around the guide. Not brilliant unless it's compared to std. What that did was to make it the most powerful Spitfire even with lower 9-1 compression pistons rather than the previous 10.5 and 10-1 in earlier models. They could have done the same to all the Lightnings because the engines were otherwise the same except for the number prefix. It would have cost them nothing. But not the way they rolled, they probably didn't want the old vibrator burning off the new triple they wanted everyone to buy.
 
acotrel said:
I use 34mm Mk2 Amals on 30mm ports with the first 10mm of the ports tapered. I don't know why I did that, but I must have had a reason. I probably think about Commando engines a bit differently. When I first bought the 850, I thought that being a Norton, with their racing history - it would probably be OK. But the crank lookes horrendous, and I never really believed in it until I started to develop it. I trusted the part of the inlet port from the valve guide to the valve seat to be pretty correct. From experience I knew what happens when the inlet ports are too large. After tuning and gearing, I now believe the rod to stroke ratio to be very good.
Top end power is not so relevant as some people might think , as long as the motor pulls very strongly, somewhere within its usable rev range. The gearbox is the torque converter. Dynos run on torque. Because I have not got a dyno, I choos an ignition advance to suit the fuel ans the compression ratio, then jet to get the fastest acceleration on a race circuit. If you have a dyno, you can get the jetting pretty right using an oxygen probe then while the bike is running on the dyno, adjust the ignition timing to get maximum torque.
If you alter the port shape during development, you are almost back to square one. In most cases, you cannot add metal inside the port.
Another thing to consider is at TDC on the non-compression stroke, the inlet and exhaust valves are both open, and the exhaust and inlet tracts affect each other. Fitting a megaphone to an exhaust, in place of a silencer without retuning can burn a piston.
Click to expand...
I took my BSA to run on the dyno but no bigger jets, I was thinking about moving the timing but it was so lean he said don't worry about these numbers it is way too lean. I haven't got an A/F gauge on this, but I should fit one and set the carbs properly. I fitted richer jets after, but that is a guess. It has Norton crank halves bolted to a new centre at 90degree. The rod length is slightly longer than a Commando rod but probably only a mm. That makes it breath better than longer rods slightly but also vibrate more and adds side load. Longer would be better to hold rpm and last. The best we saw was 85rwhp at around 8,000 as the engine rpm was not counting properly and all over the place but it didn't sound more than 8. It needs 12-1 comp to respond like the smaller motors or the inlet runners need reducing. If the inlet gas speed is increased it would have more compression because more charge would go in. That's what is happening with your smaller ports.

 
Here's my fear of going to get my engine dyno'd.

Who operates the throttle, the bike owner, or the owner of the dyno? 8000 RPM would be at the hairy scary edge. I'd prefer to only go to around 7000 RPM.

I don't have a tach to watch and really don't want to be picking up pieces of my cases off the shop floor after the big ends let go with somebody else twisting the throttle.
 
Fear not… A dyno has a lead to hook up onto your HT lead or coil to trigger their electronic tacho function. And you tell the guy what rev limit to use.
 
I knew about the tach function on the dyno, I asked if a positive ground was an issue last year when I was thinking about getting the engine dyno'd. Canceled that thought when my gearbox started acting up and my clutch started slipping WOT. However, one never knows about the guy twisting the throttle. Here in Seattle the operator could be vaping bubble hash during lunch hour. He might be looking down for a tachometer and not finding one until the engine was at 8000 RPM. :)

The gearbox and clutch are no longer issues, so going to do it this summer. Probably be a real eye opener. Like 45HP at 6200 RPM for all the money I've thrown at it. Doesn't matter. Bolting all that stuff up gave me something to do.
 
I find it hard to believe the average commando (or even a 90 degree crank) can even get to 8000 rpms unless it's a short stroke engine. It would be a contest to see whether the piston speed makes it blow up before the valve float.. 🤣 When I am doing a speed run, I duck my head down behind the fairing and I can hear the engine is making that sound like it's going flat out.... The sound I expect to hear after that sound is "BOOM"....
 
Triton Thrasher said:
Or is atmospheric pressure “blowing” it in?
Click to expand...
Yea I don't always know what I'm talking about, but the entry of the air is caused by the low pressure generated further in, not by pushing the air into somewhere the air has to fight its way. I could be totally wrong, but the air goes in because of the pull on it and the edge would indeed disturb that, it does through a carb as well and cfm is decreased. But the basic thing is a bigger carb can flow more air despite the edge. If it breaks the air on the edge it blocks around that tear and it speeds the air down the centre so that may get more into the cylinder possibly. But that defeats as it may enable. It basically needs the biggest quantity of air at the highest speed to over fill the cylinder before the inlet shuts. The 883 would make more hp if the flow quantity stays the same but is sped up through a smaller port runner and the ideal would be closing the runner down and stopping before it starts decreasing the cfm. I haven't done that on that head, but I have on others. That would peak the power earlier and move it all down in rpm and remove flat spots and waiting. As the gas speed fills it more at lower rpm and uses the air, if you can follow that. It's only filling 111% capacity at 6,000 and it is possible to get 127%. It hasn't gone over the peak near 8,000 but if it burns the same quantity of charge at 7,000 it makes the same power though the actual dynamic compression is increased. I would have loved to work on engines that I didn't have to pay for. But it is fun all the same. All enabled by 4 old vacs and $150 worth of polly pipes and plastic tubes and rulers. And seeing Jim's post showing pictures of the XR750 port.
 
Last edited:
o0norton0o said:
I find it hard to believe the average commando (or even a 90 degree crank) can even get to 8000 rpms unless it's a short stroke engine. It would be a contest to see whether the piston speed makes it blow up before the valve float.. 🤣 When I am doing a speed run, I duck my head down behind the fairing and I can hear the engine is making that sound like it's going flat out.... The sound I expect to hear after that sound is "BOOM"....
Click to expand...
My 850 motor is almost stock. I redline it at 7000 RPM, but I often see about 7300 RPM on upchanges. It would go to 8000 RPM easily and probably stay together - the main thing is crank balance factor. The crank must be balanced so it does not destroy the motor when revved high. Triumph 650 valve gear can run to 10,500 RPM if lightened and the cam is gentle. Norton valve gear is similar.
With a 400cc cylinder size in a Commando engine and a 30mm port size, it is almost impossible to make the motor gasp by whacking the throtttle open after you have changed up, even when using lean needles. The strength of a Commando motor is in it's smooth pulling power at relatively low revs - 6500 RPM is low. To use it effectively, the bike needs to be set-up very differently to most others.
My problem was, when I got the Seeley 850 going properly, I had to teach myself how to use it. It is very different
 
If you raise the crank balance factor to a sensible level, the motor will usually spin up to a higher level and do it quicker. The low balance factor was not introduced to improve performance. I would think a 750cc Atlas would be quicker than a 750cc Commando, it both were unmodified.
 
mark parker said:
Yea I don't always know what I'm talking about,
Click to expand...
Don't worry about it, you are in the vast majority!! I don't know about all our friends in the old colonies, but here in the UK maths and the sciences stopped being taught beyond the basics several decades ago when league tables and targets were introduced into our education system . When writing technical articles and giving lectures I quickly learned to not include calculations as the audience generally preferred the story and were turned off by the maths.

Getting back to the design of intake systems, a considerable amount of work has been done on the shape of the entry, lengths, diameters, port shapes, surface roughness, optimal gas velocity etc. some of which can be found on the internet, or in books.
 
o0norton0o said:
I find it hard to believe the average commando (or even a 90 degree crank) can even get to 8000 rpms unless it's a short stroke engine. It would be a contest to see whether the piston speed makes it blow up before the valve float.. 🤣 When I am doing a speed run, I duck my head down behind the fairing and I can hear the engine is making that sound like it's going flat out.... The sound I expect to hear after that sound is "BOOM"....
Click to expand...
That motor is rubber mounted so it feels happy enough, When I went from ridged mount to rubber mounts instead of feeling terminal it felt like it was loving the rpm, the Dyno operator asked if he could run another thousand rpm to see where the power went over, but I wanted to ride it home. When I have the head off, and I'm in no hurry, I'll try moving that power down in rpm and maybe fit decompressor buttons. The 750 short strokes could rev but don't need to go past 7,500 the big one feels lazy in comparison. I guess it would be fun to have the 883 kick in like them. I have the std 74mm stroke cranks cut and bolted at 90degree they are quite smooth ridged mounted.
What I have found in relation to Jim's post is an engine responds to cfm and the speed of the inlet charge. That speed is determined by compression and cam events and pipe size and design. But the inlet ports are the fundamental thing. Fitting these inserts probably do not reduce flow (cfm) just speed it up because the tube is smaller. That speed pushes or carries the charge in against the rising piston just before the valve closes so the volumetric efficiency goes up and the hp goes up without increasing rpm and it's nicer. Punchier.
 
Schwany said:
I'm working on an early 750 Commando head I bought new in the 90's for grins. No clue what the R number is. Intake ports are 28mm. I plan to open it up some, but not a lot. Doing other shaping things to it using hints from your race manual and some of my own ideas to improve oil flow into the pushrod tunnels for the tappets and also back into the crank case. Not sure I'll ever use it, but just in case it will be ready.
Click to expand...
Don't open it up .
 
Torontonian said:
Don't open it up .
Click to expand...
If I was doing it for you, I would listen to that advice, but I'm doing it for myself. LOL

I know a 31mm intake port entrance works with my engine, carburetion, ignition, and exhaust configuration. Fred Barlow who knew plenty about porting British motorcycle heads did the head I'm using now. The intake entrances are 31mm. I am stopping at 30mm on this head. It is still necked down to around 29.5mm until it reaches the guide and bowl where it opens up again. I have no intention of ruining the head. I'm my own expert. Really.
 
hybridracer said:
Don't worry about it, you are in the vast majority!! I don't know about all our friends in the old colonies, but here in the UK maths and the sciences stopped being taught beyond the basics several decades ago when league tables and targets were introduced into our education system . When writing technical articles and giving lectures I quickly learned to not include calculations as the audience generally preferred the story and were turned off by the maths.

Getting back to the design of intake systems, a considerable amount of work has been done on the shape of the entry, lengths, diameters, port shapes, surface roughness, optimal gas velocity etc. some of which can be found on the internet, or in books.
Click to expand...
I trust Joe Craig. The Norton twin racers came after the Manx, and Poms do not change things just because they can. The shape of the ports just before the inlet valve, probably has a lot of effect, and I would not change it on a whim. There would need to be a strong design argument. Most old road racers probably know about port diameters. When I was 18, I knew everything. Baldo Meli used to work for the Triumph factory and got a 12th at the IOM in about 1949. When he said 'SU carburettors are good', I thought he was an idiot.
 
About what is taught in the colonies. When I was in high school, I could always do the maths in classes, but I failed exams. I always had two jobs after school to get moneyto buy old motorcycles. Of an evening, I would walk a mile in either of two directions to where my mates lived and we would build road bikes into racers for use on public roads. All of my tertiary qualifications were achieved through part-time study while working, and I only ever applied for jobs which I was interested in doing. I have done 3rd year uni maths and physics. When I was doing my first diploma, it took me 3 times to pass statistics, because it was rule based. But for me, that became the most useful subject. When combining chemistry with computers, matrix algebra and statistics are the go. It might sound complicated, but it is actually dead easy - there is just a lot of it. In one job I had, my boss came to see how I did chemical analysis of steel after I had been there for ten years. I pushed the button and he watched it happen. He was horrified - said ' I know what I don't want to know' , and fled. But he came back later and thanked me. He was the big stand-over rugby guy - I love them. When I work, I always have fun. If I had been smart, I would have enticed him into pushing the button - it used to give a real power trip.
Most of my job training was done by Jews and Nazis in the Commonwealth Analyst's Branch of the Customs Department, and in the Government Aircraft Factories by ex- air force people. I have had about 10 career changes. After I left Customs, I was always in charge of my workplace.
 
I would imagine a lot of ideas that ended up working out extremely well in the olden days started on a whim and a drawing on a napkin in a bar or a scratch pad on the drawing board or work bench. The continued engineering, testing and finally technical specifications came after the implemented idea (whim) showed some level of improvement. One man's whim is another man's success story.
 
acotrel said:
My 850 motor is almost stock. I redline it at 7000 RPM, but I often see about 7300 RPM on upchanges. It would go to 8000 RPM easily and probably stay together - the main thing is crank balance factor. The crank must be balanced so it does not destroy the motor when revved high. Triumph 650 valve gear can run to 10,500 RPM if lightened and the cam is gentle. Norton valve gear is similar.
With a 400cc cylinder size in a Commando engine and a 30mm port size, it is almost impossible to make the motor gasp by whacking the throtttle open after you have changed up, even when using lean needles. The strength of a Commando motor is in it's smooth pulling power at relatively low revs - 6500 RPM is low. To use it effectively, the bike needs to be set-up very differently to most others.
My problem was, when I got the Seeley 850 going properly, I had to teach myself how to use it. It is very different
Click to expand...
If your 850 uses the stock cam then you are revving for lower power by taking it to 7300 rpm.
The stock cam hits max bhp at 5900.
A shift at somewhere around 6200 seems to give max propulsion with the stock cam as the rpm at start of the next gear figures into the equation.
7300 is definitely way past max power for an 850 Commando, even with most of the bigger cams. I believe most of those find max bhp around 6500.

Glen
 
This bike is basically stock except for the head and carbs and the tallest gearing I could fit at the time, a 21t gearbox sprocket. (It now has a special 22t but 1st and 2nd really need the C/ratio gears so they are tall enough to push against and move the bike). It has a roller bearing timing side main, but the basic engine is a std BSA 650 tune wise, std compression 9-1, cam std Firebird pipes and mufflers. And a ported Thunderbolt head with 2 34mm pwk carbs that cost about $70 the pair. It vibrates so bad you can hardly hold the bars by 6,000 and I think it is going to a max of about 7,500 in second gear and being rolled off a bit esp in higher gears. BSA management chose this vibration level when they decided to balance the things at 70% rather than 55-60% their paid their staff recommended after extensive testing. BSA paid for it and used around 10 engines, then chose not to employ it. (Those bosses would not have been drawing on napkins unless it was fiscal stuff, definitely not too interested in learning and discovering stuff about improving motorcycles, after all they were the chiefs.) So, this is the difference just the inlet port breathing makes to what BSA sold. The two poplars are about 1/8 mile from where it starts. The thing has a std front wheel that leans to the left because they moved the engine to the right so the left peg isn't way out, but that moved the rear chain run so the rear wheel is a bit to the right and to line up contact patches they moved the front to the right by leaning the wheel in the forks, some owners see this and start chasing what they think is bent but the axle hole is machined to create the lean. It doesn't handle bad particularly, but this yaws a bit with hard acceleration though it seems fine at speed. People have to want something to get something. Drawings on scraps of paper is a good start. Yamaha were racing flat track with the 650 twins, enlarged to 750 and not winning, US flow specialists told them what was needed in a modified head casting. They spent millions knowing what winning meant and also knowing how much they would learn doing it.

 
mark parker said:
This bike is basically stock except for the head and carbs and the tallest gearing I could fit at the time, a 21t gearbox sprocket. (It now has a special 22t but 1st and 2nd really need the C/ratio gears so they are tall enough to push against and move the bike). It has a roller bearing timing side main, but the basic engine is a std BSA 650 tune wise, std compression 9-1, cam std Firebird pipes and mufflers. And a ported Thunderbolt head with 2 34mm pwk carbs that cost about $70 the pair. It vibrates so bad you can hardly hold the bars by 6,000 and I think it is going to a max of about 7,500 in second gear and being rolled off a bit esp in higher gears. BSA management chose this vibration level when they decided to balance the things at 70% rather than 55-60% their paid their staff recommended after extensive testing. BSA paid for it and used around 10 engines, then chose not to employ it. (Those bosses would not have been drawing on napkins unless it was fiscal stuff, definitely not too interested in learning and discovering stuff about improving motorcycles, after all they were the chiefs.) So, this is the difference just the inlet port breathing makes to what BSA sold. The two poplars are about 1/8 mile from where it starts. The thing has a std front wheel that leans to the left because they moved the engine to the right so the left peg isn't way out, but that moved the rear chain run so the rear wheel is a bit to the right and to line up contact patches they moved the front to the right by leaning the wheel in the forks, some owners see this and start chasing what they think is bent but the axle hole is machined to create the lean. It doesn't handle bad particularly, but this yaws a bit with hard acceleration though it seems fine at speed. People have to want something to get something. Drawings on scraps of paper is a good start. Yamaha were racing flat track with the 650 twins, enlarged to 750 and not winning, US flow specialists told them what was needed in a modified head casting. They spent millions knowing what winning meant and also knowing how much they would learn doing it.

Click to expand...

It is unusual for a balance factor to be too high. When that happens, usually the vibe transfers to the bottom part of the rev range as a strong low frequency pulse. Sometimes high comp. pistons have heavy crowns. When you think about what happens with a crank as it spins - the pistons and rods get heavier as the crank spins-up until their inertia matches that of the counterweight. At low revs, you feel the extra mass of the counterweight. When the balance factor is too low the motor internals probably fight each other - the vibration energy has to go somwhere. The 54% commando factor actually worked well with the isolastics for a road bike. But I will bet the John Player Nortons always had high balance factors. Most 650 Triumphs perform well, however I suspect the Triumph Saint 650 was made for traffic wardens and road side assist people. It has the light one-piece crank and is almost vibrationless. It doesn't perform like any other 650 Triumph. I rode one years ago,it was hopeless.
 

Similar threads

elefantrider
Intake Manifold Cap Screws
Replies
10
Views
323
ilikevelos
I
D
Inlet manifold balance stubs.
Replies
17
Views
538
Schwany
Schwany
Anvill
Mikuni single manifold inner bolts
Replies
4
Views
466
Larry S
Larry S
jseng1
More power with more intake length
2 3
Replies
46
Views
3,431
acotrel
acotrel
jms
1967 Atlas Carbs and Manifolds
Replies
3
Views
315
marinatlas
marinatlas
Back
Top